Biorefineries could become the foundation of industrial development in the twenty-first century. The biorefinery is similar in concept to the petroleum refinery, except that it is based on conversion of biomass feedstocks rather than crude oil. Biorefineries in theory can utilize multiple forms of biomass to produce a flexible mix of products, including chemicals, fuels, power, heat, and materials.
The biorefinery concept has already proven successful in the global agricultural and forest-products industries, where such facilities now produce food, feed, fiber, or chemicals, as well as heat and electricity to run plant operations. Biorefineries have long been in place in the pulp and paper industry, wherein hardwood or softwood is converted into pulp for papermaking and other uses. Currently, the high processing costs and the narrow margin between feedstock costs and product value are important obstacles to commercialization beyond these traditional industries.
The growth of the biorefining industry relies on the efficient conversion of not just wood, but many other types of lignocellulosic biomass which are abundantly available annually. Examples of such lignocellulosic biomass include hardwood, softwood, recycled paper, waste paper, forest trimmings, pulp and paper waste, corn stover, corn fiber, wheat straw, rice straw, sugarcane bagasse, and switchgrass. Efficient conversion includes overcoming one of the key technical challenges for the emerging biorefining industry: the recalcitrance of the cellulose contained in naturally occurring lignocellulosic biomass. Overcoming the recalcitrance of cellulose so that it can be depolymerized to glucose is important, as glucose is a biorefinery platform intermediate that can be fermented or reacted to a wide variety of industrially relevant chemicals, such as ethanol, citric acid, and the like.
Lignocellulosic biomass typically contains 35-50 wt % cellulose, 15-35 wt % hemicellulose, and 5-30 wt % lignin, depending on its origin (Zhang and Lynd, 2004; Klein and Snodgrass, 1993; Wyman, 1994). Although cellulose, hemicellulose, and lignin are usually the major components of lignocellulosic biomass, there also exist varying amounts of other materials present in both bound and unbound forms. These minor components include proteins, uronic acids, acetic acid, ash, free sugars such as sucrose, soil, and foreign materials such as metals originating from harvest operations.
Cellulose is nature's most abundant polymer and is a polymer of glucose. The glucose molecules are joined by β-1,4-glycosidic linkages which allow the glucose chains to assume an extended ribbon conformation. Hydrogen bonding between chains leads to the formation of flat sheets that lay on top of one another in a staggered fashion. As a result, cellulose is very chemically stable and serves as a structural component in plant walls (Paster et al., 2003).
Hemicellulose is a polymer containing primarily 5-carbon sugars such as xylose and arabinose with some glucose and mannose dispersed throughout. Hemicellulose forms a polymer that interacts with cellulose and lignin in the plant wall, strengthening it.
Lignin helps bind the cellulose-hemicellulose matrix while adding flexibility. The molecular structure of lignin polymers is random and disorganized and consists primarily of carbon ring structures (benzene rings with methoxyl, hydroxyl, and propyl groups) interconnected by polysaccharides.
The recalcitrance of lignocellulosic biomass is believed to be caused by (i) the complicated linkages among several main polysaccharides—cellulose, hemicellulose, and lignin, which restrict the hydrolysis action of cellulases, hemicellulases, and laccases; and (ii) the inherent properties of cellulosic material—low substrate accessibility to cellulases, high degree of polymerization, and poor solubility of cellulose fragments in water (Zhang and Lynd, 2004). The lignin-hemicellulose matrix encases cellulose and prevents access of cellulase enzymes to the cellulose phase. Cellulose and hemicellulose in native lignocellulosic biomass are only slightly digestible by cellulase and hemicellulase enzymes.
Pretreatment of lignocellulosic biomass has been an actively researched field for several decades, and a wide variety of thermal, mechanical, and chemical pretreatment approaches (and combinations thereof) have been investigated and reported in the scientific literature (McMillan, 1994). The objective of pretreatment, historically, has been to break up the linkages among cellulose, hemicellulose, and lignin by removing lignin and/or hemicellulose, to produce enzymatically digestible cellulosic solids. The aim has been to maximize conversion of carbohydrate polymer to the desired monomer while minimizing the loss of the desired monomer to degradation products.
Modem pretreatment approaches have evolved from traditional thermochemical biomass-hydrolysis processes that were developed prior to World War II (McMillan, 1994). These processes typically employed cooking of biomass with an acid catalyst (often hydrochloric or sulfuric acid) in a pressurized reactor to hydrolyze the cellulose fraction of biomass to glucose. In such processes, yields of glucose are typically no higher than about 60%, as the harsh conditions required for cellulose hydrolysis result in a significant fraction of the released glucose being converted to non-fermentable sugar degradation products such as 5-hydroxymethylfurfural. In addition, single-stage processes designed for cellulose hydrolysis resulted in the loss of pentose carbohydrates (C5 sugars) from the hemicellulose fraction.
The discovery of cellulase enzymes and the subsequent development of an industrial cellulase industry, coupled with the availability of efficient pentose-fermenting microorganisms, have dramatically altered the way in which the pretreatment of biomass is approached. Rather than requiring a thermochemical process to hydrolyze cellulose to glucose, the aim of many pretreatment approaches is to produce a solid substrate in which the cellulose can be efficiently digested (depolymerized to glucose) by cellulase enzymes.
Pretreatment of lignocellulosic biomass is often the most costly step in an overall conversion process, and it impacts the cost of most other operations including the reduction in size of the feedstock prior to pretreatment, as well as enzymatic hydrolysis and fermentation after pretreatment. Pretreatment can be strongly associated with downstream costs involving enzymatic hydrolysis, power consumption, product concentration, detoxification of inhibitors, product purification, power generation, waste-treatment demands, and other process operations (Wooley et al., 1999; Wyman et al., 2005).
Intensive lignocellulose-pretreatment efforts have been undertaken during the past several decades, but current technologies have not yet been commercialized on a large scale due to high processing costs and great investment risks (Wyman et al., 2005). Many pretreatment technologies employ severe reaction conditions resulting in degradation of sugars and formation of inhibitors, and generally high processing costs.
In general, there is good agreement in the art that amorphous cellulose is more digestible than crystalline cellulose. Hydrolysis of amorphous cellulose requires less catalyst and shorter reaction time, and has higher sugar yields, as compared with that of crystalline cellulose. Amorphous cellulose can be regarded as a homogenous substrate with at least an order of magnitude higher reaction rate than that of crystalline cellulose hydrolysis by acids (Fengel and Wegener, 1984) or cellulose enzymes (Zhang and Lynd, 2005).
A review of the pretreatment art (Chang and Holtzapple, 2000) found that the enzymatic reactivity of lignocellulosic biomass correlates most closely with lignin content and cellulose crystallinity, which both relate to the accessibility of the cellulose. It is therefore recognized that an efficient lignocellulosic-biomass pretreatment process comprises decrystallizing part of the cellulose, rendering it amorphous, as well as removing some of the lignin from the starting material. It is also desired to fractionate the biomass such that hemicellulose sugars and acetic acid can be recovered.
What is needed is an efficient pretreatment and/or fractionation technology for lignocellulosic biomass, wherein cellulose is decrystallized, lignin is substantially removed and recovered, hemicellulose sugars are substantially removed and recovered, and wherein the process conditions for performing the reactive separation do not degrade the extracted sugars or produce appreciable quantities of inhibitors for downstream fermentation.
Another economic obstacle for the fractionation of lignocellulosic biomass is that large quantities of solvent are generally required, leading to high capital and operating costs for the plant. Therefore, what is needed is a process that can achieve the benefits characterized above, using relatively low quantities of solvent, such as solvent/solid ratios of about 5 or less.
It is further desirable that such an efficient pretreatment and/or fractionation technology would be flexible for a variety of biomass feedstocks and co-product options, and would require modest process conditions so as to be economical.